The present invention is concerned with a process for converting phytate into inorganic phosphate. In particular, it concerns such a process which can be adjuncted to conventional processes which are used to extract oil from oilseeds.
Phytate [myoinositol 1,2,3,4,5,6-hexakis (dihydrogen phosphate)] is found to varying degrees in all plants as the major storage form of phosphorus. Between 60-80% of the total phosphorus in plants is in the form of phytate. Phytate in plants is often found in the form of complexes with cations such as calcium, magnesium or potassium. The resulting complexes are sometimes called phytin. The term phytate as used herein specifically encompasses such phytin complexes. Phytate is poorly digested by monogastric animals. As a result of this, monogastric animals fed a phytate-rich diet may still suffer from illnesses caused by phosphorus deficiency. This is because the phytate phosphorus is not bio-available, and the majority of dietary phytate consumed by a monogastric animal passes through its gastrointestinal tract and is excreted in the faeces. This excretion is a particular concern in areas of intensive livestock production where excessive amounts of phosphorus-enriched manure can be environmentally damaging.
A further problem associated with the presence of phytate in foods is that it forms complexes with multivalent metal cations. This can interfere with the bio-availability of such cations to animals and humans. This can lead to metal deficiency disorders or inadequate bone mineralization, especially in the case of vegetarians, elderly people and infants.
Phytate also has the disadvantage of inhibiting various enzymes in the gastrointestinal tract, including pepsin and trypsin. It is also forms complexes with proteins preventing their digestion. For these reasons, the presence of phytate in a diet is actually anti-nutritional as it reduces the digestibility of co-present proteins.
One solution which has been proposed to solve the above problems is to convert phytate into inorganic phosphate. The phosphorus in inorganic phosphate is bio-available to monogastric animals. This decreases the phosphorus content of faeces, liberates cations previously complexed by the phytate, promotes protein digestion and prevents phytate inhibition of gastrointestinal enzymes. The conversion is known to be effected by treating the phytate either in vitro or in vivo with a phosphatase enzyme called phytase. The reaction products of this conversion are myoinositol and orthophosphate, the latter being termed inorganic phosphate in this specification.
The in vivo conversion is carried out by adding phytase to foods which contain phytate. As a result, both the phytate and phytase are co-present in the gastrointestinal tract where, in theory at least, the phytase can convert the phytate into inorganic phosphate. However, this has proven to be only partially effective resulting at best in the conversion of no more than 55% of the phytate-phosphorus into inorganic phosphate, and usually a significantly smaller proportion. This incomplete conversion is primarily a consequence of the conditions within the gastrointestinal tract being quite different from those which are optimal for phytase activity. The temperature, pH, moisture and mineral content of the digesta are such that phytase is only partially effective in the gastrointestinal tract during the time which it takes for the digesta to pass through it.
The second solution of subjecting phytate-containing foods to in vitro hydrolysis with phytase has been found to be more a effective than the in vivo conversion described above. This is because the conditions of the in vitro reaction can be tailored to those which result in the phytase having its optimum activity. EP-A-0 380 343 describes one example of such a process in which phytate present in soy protein isolates is converted into inorganic phosphate. The conversion is carried out in an aqueous solution using a bacterial phytase at a pH of 2-6 and at a temperature of 20-60.degree. C.
However, it is found that even such treatments are still unsatisfactory. Firstly, the slurry resulting from these treatments has to be dried by driving away the significant amounts of water which are conventionally included. Although such drying is a relatively simple process step, it is nevertheless relatively expensive to carry out due to the bulk of water which has conventionally been used. Such a bulk is necessary firstly to provide the aqueous environment required by the phytase in order for it to be catalytically active, and secondly to facilitate mixing of the slurry which otherwise would form a relatively viscous mass. As a result of this drying problem, such in vitro processes have had limited commercial success. The second problem which has been found is that the conversion of phytate into inorganic phosphate in these in vitro processes is still far from complete unless extremely high concentrations of (relatively expensive) phytase are used. The present inventors have found that this is due to phytate existing in two forms; a phytase-susceptible form and a mineral-bound, phytase-resistant form. The phytase-resistant form has been found to be phytate in the form of a complex with alkaline earth metal cations such as Mg.sup.2+ and Ca.sup.2+.